A multiple-input multiple-output (Multiple-Input Multiple-Output, MIMO) technology is one of key technologies in the 802.11n standard protocol. In one aspect, the technology can provide transmit (receive) beamforming, thereby effectively improving transmit (receive) power, and effectively improving reliability of a communications system; in another aspect, the technology can generate an additional spatial degree of freedom, thereby exponentially increasing a system throughput, and effectively improving a rate of the communications system.
In an application process of the MIMO technology, when link antenna quantities are asymmetrical, that is, a quantity of antennas at a transmit end is not equal to a quantity of antennas at a receive end, a maximum quantity of transmitted data flows that can be supported by the MIMO technology is limited to the smallest value that is not greater than the quantity of transmit antennas or the quantity of receive antennas. Therefore, asymmetrical antenna quantities cause a degree of freedom of the MIMO technology not to be effectively utilized, and limits an increase in the system throughput. For example, in a wireless local area network, considering costs and a size, only one antenna is generally mounted on a terminal. However, an access device (Access Point, AP for short) manufacturer mounts several antennas on most current AP products. When the AP performs data communication with the terminal, if only one antenna is mounted on the terminal, a maximum of one data flow can be transmitted regardless of a quantity of antennas at the AP end.
To further increase the system throughput and the MIMO degree of freedom, after 802.11n, the Institute of Electrical and Electronics Engineers (Institute of Electrical and Electronics Engineers, IEEE for short) Union is drafting and revising the 802.11 ac standard. The standard is exclusively for a 5 GHz frequency band, a supported system bandwidth is increased from 40 Mbit/s in 802.11n to 80 Mbit/s, and even to 160 Mbit/s. The standard supports a higher-order 256 QAM modulation mode, and supports simultaneous transmission of a maximum of eight flows.
Considering a limitation of asymmetrical link antenna quantities to the MIMO technology, a multi-user multiple-input multiple-output (Multi-user Multiple-Input Multiple-Output, MU-MIMO for short) technology is introduced in the 802.11ac standard. By using the MU-MIMO technology, a transmit end can simultaneously communicate with multiple receive ends, and a quantity of data flows that can be simultaneously transmitted by the transmit end is not greater than the smallest value of a quantity of antennas at the transmit end and a sum of quantities of antennas at the receive ends. Therefore, when there are enough receive ends, even if there are few antennas at each receive end, the degree of freedom of the MIMO technology can still be fully utilized, thereby effectively improving the system throughput.
In a wireless local area network (Wireless Local Area Network, WLAN for short), referring to FIG. 1, in a process of communication between an AP and three stations (Station, STA for short), the AP sends data packets by sending very high throughput physical layer protocol data units (Very High Throughput Physical Layer Protocol Data Unit, VHT PPDU for short) to the three STAs by using the MU-MIMO technology according to the 802.11 ac standard. When receiving the VHT PPDUs, a STA1, a STA2, and a STA 3 respectively obtain the data packets of the STA1, the STA2, and the STA 3 by means of decoding, and sequentially wait one short interframe space (Short Interframe Space, SIFS for short) to return acknowledgment frames (Acknowledgement, ACK for short) to the AP, to indicate that the STAs have correctly received the data packets sent by a transmit end. It can be learned from FIG. 1 that a total time for returning the ACKs by the STA1, the STA2, and the STA 3 is 3×(TACK+SIFS), where TACK indicates ACK sending duration.
In the foregoing communication process, the inventor finds that the prior art has at least the following defects: When a transmit end communicates with multiple receive ends, a system requires a relatively long time to return ACKs, which occupies a large amount of channel time, and results in a waste of channel resources; in addition, because returning the ACKs takes too much time, channel reliability is affected, and consequently reliability of precoding performed by the transmit end is affected, and a packet error rate is increased.